UC Berkeley

This Thesis constitutes a set of studies of seemingly disparate materials, which are united by their proximity to a type of putative quantum spin liquid ground state. A quantum spin liquid (QSL), is an exotic magnetic phase which can exist in materials that have strong magnetic exchange interactions amount isolated spins, but do not magnetically order due to a large degree of magnetic frustration induced either by the geometry of the lattice or the inequivalence of the bonds. This phase is characterized by the emergence of fractionalized quasiparticles, the spectrum of which can be gapped or gappless. In the gapless case, it is possible for these quasiparticles to contribute to a charge-neutral Fermi surface. In the gapped case, it is possible to induce a fermionic spectrum with nontrivial band topology, leading to exotic thermal transport signatures. In the most exotic cases, these quasiparticles can acquire fractional statistics and potentially non-Abelian exchange, which could be extremely useful for the field of topological quantum computation.

Unfortunately, it is extremely hard to prove the existence of a spin liquid, as its properties do not lead to obvious signatures in standard experimental probes. That said, even when a system is close to the QSL ground state, it can lead to a plethora of interesting physical phenomena, particularly in the realm of unconventional superconductivity and magnetism.

We first state our results for the material 4Hb-TaS$_{2}$. This material is an unconventional superconductor and thought to be adjacent to spin liquid physics due to the 1T-TaS$_{2}$ layers within its bulk. We perform thermal conductivity measurements of 4Hb-TaS$_{2}$ as a function of field and temperature. We observe a deviation from the Wiedemann-Franz law at low temperature caused by an upturn in $\kappa/T$. We speculate that this could be due to the presence of charge-neutral quasiparticles from the proximate spin liquid state. In the magnetothermal conductivity, we observe a remarkable hysteretic behavior manifesting as downward spikes in $\kappa$ when field is applied parallel to the $c$-axis. This hidden magnetic phase may have also been observed in previous scanning SQUID measurements, but the signatures were much more subtle. Notably, no hysteresis is observed in the resistivity, suggesting that this magnetic phase does not couple to the charge degrees of freedom. These results are discussed in Chapter 3.

We then discuss our measurements of the high-field magnetic phase diagram of \blio{} using the technique of resonant torsion magnetometry (RTM). \blio{} is a Kitaev spin liquid material with a rich magnetic phase diagram. The system orders magnetically at 38 K with an incommensurate counterrotating spiral structure. We observed the phase boundary at magnetic fields up to 60 T for the first time, in line with theoretical results, we also observe a saturation field $H^{**}_{ac}$ within the $ac$ plane, as outlined by theory. Finally, we find a finite-temperature high-field phase with field applied along the $c$-axis, originating from symmetries that remain unbroken after transitioning out of the incommensurate phase. These results are discussed in Chapter 4.

Finally, in Chapter 5 we discuss our preliminary measurements of 1T- and 2H-TaS$_{2}$. As previously mentioned, 1T-TaS$_{2}$ contains a putative quantum spin liquid ground state. Both polytypes have a very low magnetic anisotropy and a poorly characterized low-temperature magnetic phase diagram, leading us to use RTM for this set of materials as well. We observe very unusual angle dependences in both polytypes. The 1T polytype exhibits signatures of time-reversal symmetry breaking at low fields, along with an unexpected phase shift in the angle dependence of the magnetotropic susceptibility, which is quite surprising. We also observe an additional $\pi$-periodic component in 2H-TaS$_{2}$, the origin of which is unknown. Finally, both the 1T and 2H compounds exhibit strange enhancements in the magnetotropic susceptibility at various angles in low magnetic fields, which could indicate proximity to a nearby phase transition.